FIELD OF THE INVENTION
The present invention concerns a superstructure for a very high power cell for the production of aluminium using to the Hall-Heroult process by the electrolysis of alumina in molten cryolite. This cell superstructure is designed to support the different devices connected thereto which are vital for the operation of the cell and are thus of necessity situated in its immediate environment, while taking up the least possible amount of space and causing minimum obstruction.
The superstructure of a modern electrolysis cell is formed by one or more horizontal beams made of steel, the ends of which are supported upon legs, and which support the devices connected thereto comprising the anodic current risers and the anodic frame structure formed by aluminium bars to which the anodes are connected, the cryolite and alumina feed systems (alumina reservoir, crust breaker, distributor-metering system), the mechanisms for controlling upward and downward movement of the anodes, and in many cases, ducts for gathering effluents, gas and dust emitted by the tank.
The legs rest upon the ends of the metal casing which forms the electrolysis cell in the true sense. This arrangement has the advantage of making space available at the two long sides of the cells by way of which operations of changing anodes are effected.
The present tendency is to provide for a constant increase in the unitary power of the cells which results in an increase in the length of the casing, which length may exceed 15 meters for very high power tanks operating at over 300 kA.
In this case the construction of the superstructure raises a difficult problem as it must have sufficient rigidity to:
support with its own weight the anodes and all the devices connected to it as set out above, PA1 support the force necessary to break the crusts formed by solidified electrolyte and which resist the vertical movements of the anode, PA1 ensure that the anode-cathode distance (about 40 mm) is constant over the entire length of the cell as the procedure for regulating the cells requires extremely accurate positioning of the anodic plane with respect to the horizontal cathodic plane formed by the layer of liquid aluminium. PA1 maintain sufficient rigidity of the horizontal beams despite the increase in the weight of the anodes and of certain devices connected thereto, and to do this without increasing the height of the superstructure, which would necessitate raising the height of the building, PA1 to effect anode changes without the difficulty resulting on the one hand from obstruction and risks of loss caused by the removable parts of the connectors, and on the other hand by the insufficiency of the multi-piered structure with the type of anode rod connection and to do this without elongation of the superstructure or increase in the centre distance between cells, and thus without modification of size at ground level in buildings.
In order to do this the thickness and the height of the beams, and consequently the mass thereof are increased. The increase in height has repercussions upon the height of the buildings and thus upon cost. In consequence, this line of development very quickly becomes limited.
For high power cells which nevertheless operate at below 300 kA, a solution is to be found in the addition of supporting points or intermediate gantries between the legs at the ends supporting the horizontal beams, such as specified in EP-A-0210111 (U.S. Pat. No. 4,720,333) to form a superstructure with multiple supports or a multi-piered structure.
For very high power cells operating at over 300 kA this solution cannot really be applicable because of the impossibility of performing certain maneuvers, particularly during changing of the anodes, and the risk of accidents caused by the excessive obstruction along the long sides of the cell.
Indeed, with the increase in the number of anodes, but particularly with the increase in their size and therefore of their unitary mass, which exceeds 2 tons, new difficulties arise in achieving a good yield from the cells, which particularly call into question the multi-piered superstructure with legs at its ends and intermediate gantries such as described above. This superstructure has shown itself to be incompatible with certain integrated devices, rendering inaccessible, for example, electrical and mechanical connection systems, or connectors, of the anode rods onto the anodic frame structure.
Therefore, the connectors normally used on medium and high power electrolysis tanks (l&lt;300 kA) are "straight entry" connectors such as those described in U.S. Pat No. 3,627,670 (FR-A-2039543) by which the positioning before clamping of the anode rod onto the anode frame structure is effected by movement of this rod, maintained in a vertical position, towards its location in the connector along a plane perpendicular to the plan of the anodic frame structure. With the increase in size of the anodes this manoeuvre for approaching the connectors near the support points, that is to say the legs at the ends and the intermediate gantries, becomes impossible as these supporting points are situated in the trajectory of the anodes.
Moreover, these "straight entry" connectors comprise a fixed part firmly attached to the anodic frame structure which ensures centring of the anode rod, and a removable part which ensures clamping and blocking of the rod against the anodic frame structure once it is in position. With regard to holding anodes the mass of which is in excess of 2 tonnes, the quality of the contact and the clamping of the anode rod against the anodic frame structure has to remain excellent in order to limit the difference in potential at the rod/frame interface, and also to avoid any sliding of the anode and thus any disturbance due to a local variation in the anode/cathode distance. In order to do this, the clamping pressure and therefore the size of the connectors, and particularly of their removable part, has to be increased considerably. During changing of the anode, hanging means need to be provided for temporarily holding those removable parts, the mass of which can reach 30 or 40 kg, thus increasing the risks of loss and obstruction in the working area.
Having regard to these disadvantages, the inventor has developed a new superstructure in combination with another type of anode connector for high power electrolysis cells, in order to: